Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2002-07-24
2004-09-21
Allen, Stephone B. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S559400, C271S265010
Reexamination Certificate
active
06794633
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a sheet detecting device and an image forming apparatus.
2. Description of Related Art
The conventional image forming apparatus such as a copying machine, a facsimile machine and a printer for dealing with sheets are generally constructed in structure provided with a sheet transporting device for feeding (or transporting) each sheet to a predetermined position in synchronism with a feed signal from the main body of the apparatus.
In the sheet transporting device of this type, it is necessary to separate and feed sheets one by one from a stack of sheets stacked on a tray or in a cassette and transport each sheet to a predetermined position at predetermined timing synchronized with formation of an image, with a high degree of accuracy. For this reason, a sheet detecting device is disposed on the sheet transporting device to detect the position of the leading edge or the trailing edge of each sheet.
The sheet detecting devices are generally classified into a contact sensor type and a non-contact sensor type.
The contact sensors are detecting devices frequently used heretofore. For example, there are the known contact sensors of structure in which an inclinable lever is arranged to project in a sheet transport path and in which, with a transporting sheet coming into contact with one end of the lever, an inclination of the lever is detected by a transmissive photosensor provided at the other end.
As the recent increase in operation speed of printers raised the sheet transportation speed, the conventional contact sensors came to encounter a problem of damaging the leading edge of the sheet, however. There are thus increasing tendencies to equip the high-speed machines with an optical sensor for optically and directly reading the presence/absence of a sheet in a non-contact manner.
The optical sensor is also used for the purpose of discriminating a type of each sheet. For example, in the case of a color printer being configured to form a color image through multi-layer transfer steps of toner materials of multiple colors, in order to effect print on a light transmissive sheet for OHP, it is necessary to perform a control of switching a fixing speed to a lower speed enough to increase optical transmittance by sufficiently fusing the toner, and the optical sensor is thus arranged to detect whether the sheet is a light transmissive sheet such as an OHP sheet.
A conventional optical sheet detecting device used in the image forming apparatus will be described below with reference to FIG.
15
.
The sheet transporting device of
FIG. 15
is provided with a stepping motor (not shown) which drives a sheet feed load, described hereinafter, at a predetermined speed in accordance with a command from a control unit (not shown); a semicircular roller
201
which separates and feeds a sheet from a stack of sheets in a cassette
214
on the basis of a control of releasing a latch by a solenoid (not shown) and mechanically rotating the roller through one revolution; transporting rollers
202
disposed downstream of the semicircular roller
201
; registration rollers
204
which are switchable between a halt and rotation by an electromagnetic clutch; a sheet presence/absence detecting sensor
203
of the contact type as an ante-registration sensor disposed immediately before the registration rollers
204
; and a leading edge sensor
205
as an optical sheet presence/absence detecting means disposed downstream of the registration rollers.
The main body section as an image forming means of the image forming apparatus is provided with a photosensitive drum
211
as an image bearing member; a scanner
210
which forms an electrostatic latent image on the photosensitive drum
211
; a developing device
209
which develops the electrostatic latent image with toner materials of respective colors of C, M, Y, and K; a transfer drum
207
which rotates in a state in which a sheet transported by the sheet transporting device is wound around and attached onto the transfer drum
207
, and which transfers toner images of the respective colors formed on the photosensitive drum
211
, onto the sheet; a stripping claw
212
which strips the sheet with the toner images transferred thereon, from the transfer drum
207
; and a fixing device
213
which thermally fixes the transferred toner images on the sheet. A gripper
208
for gripping the leading edge of the sheet is provided on the transfer drum
207
and a gripper position sensor
206
, which detects arrival of the gripper
208
at a position equivalent to the sheet feed position of the leading edge sensor
205
, is provided in the vicinity of the transfer drum
207
.
A configuration of the control unit, which controls the hardware structure as described above, will be described. When a print signal is issued, the control unit rotates the semicircular roller
201
through one revolution to feed a sheet at a predetermined speed, and also rotates the transfer drum
207
.
The sheet transported by the transporting rollers
202
comes into abutment against a nip between the registration rollers
204
kept in a halt state to form a loop of a certain size, thereby implementing skew-feed correction. The registration rollers
204
then start to be rotated at a certain time after detection of the leading edge of the sheet at the ante-registration sensor
203
to lead the skew-corrected sheet in. When the leading edge sensor
205
detects the leading edge of the sheet thereafter, the registration rollers
204
are again brought into a halt state to stand by.
When the gripper sensor
206
detects arrival of the gripper on the transfer drum
207
, the control unit restarts the stepping motor and controls the registration rollers
204
so that the sheet is transported at a feed speed relatively faster than the speed of the transfer drum
207
for a certain period of time and thereafter the feed speed is switched back to the same speed as the speed of the transfer drum
207
.
This makes it feasible to perform such synchronous control as to close the gripper
208
while the sheet butts by a predetermined amount against the gripper
208
opening approximately 30° relative to the surface of the transfer drum
207
, and always feed the sheet stably to the gripper position as a leading edge position during the transferring operation.
The following will describe a control operation performed in feeding an OHP sheet by the sheet feed control and the optical sheet detecting device.
When an OHP sheet is fed up to the leading edge sensor
205
through the sheet feed control, the leading edge sensor
205
detects a light shield portion preliminarily printed in the width of 5 mm downstream from the leading edge on the sheet, whereupon the stepping motor is halted to stand by. When the gripper sensor
206
detects the gripper, the OHP sheet is refed. Thereafter, the leading edge sensor
205
detects a transmissive portion spaced by 20 mm and subsequent distances away from and downstream of the leading edge of the OHP sheet (or detects transmission of light) to make a judgment as an OHP sheet. Then toner images are transferred, and thereafter the control unit performs such control as to decrease the driving speed of the fixing device
213
to one third of the normal speed at the time of stripping and discharging the sheet.
A configuration of the leading edge sensor
205
will be described below referring to
FIGS. 16A and 16B
.
In
FIGS. 16A and 16B
the leading edge sensor
205
is a transmissive photosensor in which a reflecting member
126
is disposed on one side of the sheet transport path
121
and a light emitting and receiving unit
120
including a light emitting element
122
and a light receiving element
123
is disposed on the other side.
When no sheet S is present as shown in
FIG. 16A
, light L emitted from the light emitting element
122
travels through a slit
124
provided in a light shield cover to be reflected by the reflecting member
126
, and the reflected light again travels through
Allen Stephone B.
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Lee Patrick J.
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